According to the Dallas Morning News, the Cruz campaign took in $1.9 million in the last quarter of 2017. However, the incumbent Republican maintains an advantage when it comes to cash on hand, ending the year with $7.3 million -- as opposed to O'Rourke's $4.6 million, according to their report. The Cruz campaign did not immediately return CNN's request for comment.

The fundraising announcements came days before the Federal Election Commission filings deadline on Wednesday.

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O'Rourke is serving his third term in the House, representing El Paso. He launched his Senate campaign in late March 2017 and has refused to accept any corporate money or donations from political action committees -- a pledge that stretches back to his first run for the House. In an interview with CNN shortly after the start of his bid, O'Rourke acknowledged that his campaign would be seen as a long shot but said he'd "like nothing more for the establishment to count us out."

Despite his high-fundraising dollars, at least one expert doesn't think O'Rourke stands much of a chance of unseating Cruz.

Josh Blank, the manager of polling and research for the Texas Politics Project at the University of Texas, told CNN that the numbers don't "change the fundamentals" of the deep-red state. Texas has not had a Democratic senator since 1993.

"It's a real uphill climb for him," Blank said, noting that O'Rourke likely benefited from launching his campaign early and being boosted by the Democratic establishment. Despite the national recognition, Blank said O'Rourke will likely have a difficult time translating that to the local electorate. In an October 2017 University of Texas/Texas Tribune poll of O'Rourke's favorability, the majority of respondents (53%) didn't know or had no opinion of the Democrat, and 16% found him neither favorable or unfavorable.